In conventional drilling techniques, a longitudinally extending string comprising sections of drill pipe is secured to a drill bit of a larger diameter than the drill pipe. After a selected portion of the wellbore has been drilled, the drill string is removed and a string of tubular members of lesser diameter than the wellbore, known as a casing string, is placed in the wellbore. The annulus between the wall of the wellbore and the outside of the casing string is then filled with cement by pumping the cement down through a casing shoe or reamer shoe disposed at the end of the casing string.
In an alternative technique, designed to address the inefficiencies associated with making multiple wellbore trips in the conventional drilling technique discussed above, it is now known to drill with casing. In this technique, the drilling operation employs a drill bit, termed a casing bit, which is attached to the end of the casing string. The casing bit functions not only to drill the earth formation, but also to guide the casing string into the wellbore. The casing bit remains in place during subsequent cementing of the casing in place. The casing string is thus run into the wellbore as the wellbore is being formed by the casing bit. This eliminates the need for one or more extra trips to retrieve a drill string and drill bit after reaching a target depth where cementing is desired.
In either technique, additional drilling beyond the end depth of the casing string may be required. If so, the operator must drill out the casing end tool (shoe or bit) to reach the underlying formation. This is typically accomplished with a mill bit that is specifically designed to cut through the material from which the shoe is made. This has led to the development of casing end tools that are more readily drilled out. Primarily, such end tools use an aluminum alloy as the parent body material for the reamer nose or the cutting structure carrying face of the end tool. More recently, casing end tools made of alloyed steel have been commercialized and are run on casing prior to being drilled out with specially designed drill out PDC bits that carry an additional, standalone, overexposed tungsten carbide cutting structure to accomplish the drill out.
Prior art efforts relating to casing operations are set forth below. All references discussed herein are incorporated by reference.
U.S. Pat. No. 6,062,326 to Strong et al discloses a casing shoe/reamer with cutting means. The shoe/reamer has flutes (blades) that in one embodiment carry PDC cutters along the gage and across the nose of the tool. The tool is disclosed as being made either from drillable aluminum or non-drillable material. In one embodiment the nose section is designed to be segmented with the segments being hinged to the outer portion of the tool so the nose segments can be pushed out and forward prior to cementing or as part of the cementing process.
U.S. Pat. Nos. 6,401,820 and 6,659,173 to Kirk et al describe a shoe with reaming members and a nose portion of aluminum or zinc alloy to allow the nose to be drilled out.
U.S. Pat. No. 6,443,247 to Wardley describes a casing drilling shoe with an outer drilling section constructed of a hard material such as steel and an inner section constructed of a readily drillable material such as aluminum. It further includes a device for displacing the outer drilling section radially outwardly.
U.S. Pat. No. 6,848,517 to Wardley describes a drillable drill bit nozzle for use in a drill bit that is going to be drilled out.
U.S. Pat. No. 7,066,253 to Baker describes a casing shoe or reamer shoe with an outer body of relatively hard material and a nose of relatively soft material which are interlocked. A following drill bit is used to drill out the majority of the soft material leaving a sheath of the soft material in the internal circumference of the hard material.
U.S. Pat. No. 7,096,982 to McKay et al discloses a drill shoe with a body constructed of a relatively soft material which is set with blades of a relatively hard material. The blades, typically steel, are further set with PDC cutters. Once the desired depth of drilling has been achieved, a displacement element is activated to push out the soft material and bend the blades to the sidewalls of the annulus. The displacement element can then be drilled out with a following bit. McKay wants to provide a cutting structure support mechanism with the steel blades strong enough to handle drilling loads.
U.S. Pat. No. 7,117,960 to Wheeler et al describes a bit for drilling with a completion string that incorporates an integrated female non-shouldered oilfield completion string thread. The specification describes the bit as being manufactured from a material which does not allow the bit to be readily drilled.
U.S. Pat. No. 7,216,727 to Wardley discloses a casing drilling bit constructed from a relatively soft material such as aluminum, copper, or brass alloy and is coated with relatively hard material. The cutting means of the cutting members consist of fine layers or cutting elements formed from hard material.
U.S. Pat. No. 7,395,882 to Oldham et al is for “Casing and Liner Drilling Bits”. This patent teaches making such tools with an axisymmetric inner profile to be evenly addressed by a subsequent drilling bit. It also teaches using nozzles deployed with sleeves, and gage sections that extend over the casing to which the tool is attached.
U.S. Patent Application Publication No. 2007/028972 to Clark et al is for “Reaming Tool Suitable for Running on Casing or Liner and Method of Reaming”. This published application also teaches an axisymmetric inner profile and further states “. . . the absence of blades in the nose area projecting above the face of the nose allows for an uninterrupted cut of material of the body shell in the nose, making the reaming tool PDC bit-drillable.”
U.S. Pat. No. 6,845,816 to Kirk et al teaches the use of an austemperized ductile iron (ADI) material for a centralizer. This material is more robust than aluminum and lighter than and more machinable than steel. See also, for example, ADI materials provided for sale by THDick.
Reference is also made to the Baker Hughes (Hughes Christensen) EZ Case Casing Bit System and the Weatherford International DrillShoe tools used for drilling with casing prior art devices (the disclosures of which are hereby incorporated by reference).
To summarize the prior art in this area, great attention has been given to the eventual drill out of the casing end tool, but little attention has been paid to the drilling efficiency of the casing end tool itself. Significant improvements to casing end tool performance can be made by adapting efficient drilling technology to the unique challenges of casing end tool structure and architecture. The other significant trade off in the prior art is in the choice of body material. Aluminum is readily drilled out but has a low resistance to erosion and abrasion, and cannot take the level of loading that steel is able to absorb. Alternatively, steel is more robust than aluminum but is much more difficult to drill out. If casing equipment is to be drilled out with a PDC bit then this has required the use of specially designed PDC drill out bits that compromise bit performance in the rock formations encountered after drill out.
What is needed are casing end tools (including casing bits and reamer shoes, liner drill in bits, liner reamers, and liner or casing mud motor driven reamers or mills) that perform effectively while drilling or reaming, are resistant to erosion, abrasion, and impact damage, and that can be effectively and consistently drilled out using standard PDC drill bits or cutter protected PDC bits.